In the field of production and engineering, the displacement of components and devices are usually measured to characterize the characteristics of components or devices by change of the displacement or to deduce the characteristics of components or devices by combining the displacement. For example, according to the requirements on the Code for Acceptance of Construction Quality of Building Foundation in China, static load detection of bearing capacity of foundation test pile and engineering pile should be performed to determine and test the bearing capacity of single pile, which is regarded as a mandatory article for the acceptance of pile quality requirements. Now the self-balancing method is commonly used in testing the bearing characteristics of the pile foundation. The self-balancing method is that: the load box and welded steel cage are imbedded in the corresponding position of the pile in advance, and then poured into the pile. When the strength of concrete meets certain requirements, the load box will simultaneously produce thrust upwards and downwards (namely vertical direction) by pressurizing and loading the load box in the pile by the pressurized pump on the ground platform, forcing the upper and lower bodies of the pile to be displaced. The displacement of the upper and lower bodies of the pile is measured by measuring the displacement of the upper and lower surfaces of the load box, and the bearing capacity characteristics of the foundation pile is judged according to the displacement. The bearing capacity of the single pile can be calculated by the loading value corresponding to the measured displacement. However, the domestic self-balancing method often uses the method of connecting the displacement rod on the upper and lower surfaces of the load box to measure the displacement of the pile, which in some cases is not accurate and its own cost is relatively expensive, so the method is not widely used.
In the prior art, the displacement detecting device is complicated and the method is relatively cumbersome. Like laser measurement, it is possible to achieve a more accurate measurement of the displacement, but the supporting facilities are expensive, and it is difficult to be used in multiple spot and large scale, and the application is restricted greatly. Therefore, a displacement detecting device with a simple structure and accurate measurement is demanded.